Electrical and optical techniques have been applied to wideband radio frequency (RF) spectrum analysis/channelization.
A conventional electrical domain RF channelization technique employs a bank of narrowly spaced filters. For wideband applications where relatively high instantaneous bandwidth (IBW) is sought (i.e. a significant number of channels), designs become difficult to tune, bulky and expensive. An alternative electrical domain method of wideband channelization uses a high speed analog-to-digital converter (ADC) followed by a Fast Fourier Transform (FFT).
Though progress has been made in increasing ADC IBW, current ADC IBWs are inadequate to monitor the entire RF spectrum, where tens of gigahertz bandwidth is required. When the frequency range of interest is greater than the ADC IBW, the full frequency range can still be analyzed by appropriate splitting, filtering, amplifying, down conversion and digitization of the resulting multiple channels. This approach however suffers from increased size, weight, and power requirements.
Because of the limitations of the electrical domain methods, several optical domain RF spectrum analyzer/channelization approaches have been proposed. One highly studied approach employs an incoherent transversal filter. This design implements necessary negative filter coefficients, but this adds complexity to the design. There are also attendant performance degrading coherent effects that must be mitigated with this design.
The coherent transversal filter approach offers the possibility of excellent channelization performance and this approach is in fact commonly used in integrated devices for channel spacing relevant to wavelength division multiplexed optical networks, i.e., >25 GHz. For channel spacing relevant to RF applications, however, the required delay times are longer and therefore the tuning and stability of devices using the coherent approach become increasingly difficult.
Other optical domain RF channelization approaches exist such as described in R. K. Mohan, et al., “Ultra-wideband spectral analysis using S2 technology” Journal of Luminescence 127, 116 (2007) and M. Stead, “Using Dispersion in a Fiber-Optic Loop to Perform Time Domain Analogue RF Signal Auto-Correlation,” in Optical Fiber Communication Conference, OSA Technical Digest (CD) (Optical Society of America, 2011), paper OThW3.